Ophthalmic lenses may be prescribed to a user having at least one power corresponding to what the user needs to compensate for certain visual defects, of either positive or negative strength. An example of such visual defects may be astigmatism. Ophthalmic lenses may be adapted in a glasses frame depending on the prescribed power and the relative position of the user's eyes with respect to the glasses frame (vertex distance).
For presbyopic people (users with eyestrain), lenses may be prescribed with different powers (for near vision and far vision), given the difficulty of accommodation that this type of users usually have. In these progressive lenses, a power may be defined at the top of the lens corresponding to far vision, with a progressive increase of downward addition up to achieve a power for near vision at the bottom of the lens.
Optometry may be defined as the science responsible for the primary care of the visual health through actions of prevention, diagnosis, treatment and correction of defects.
In order to determine the most appropriate characteristics for glasses and, in particular, for the progressive lenses of the glasses, the user that will wear the glasses is usually subjected to an optometric study. This study may be performed using different techniques and devices aimed at that purpose, and may result in various optometric parameters associated with the user.
Typical examples of such optometric parameters may be: power for near and far vision, near and far inter-pupillary distance, working distance (which may take into account the prescribed power for near vision), vertex distance for a particular glasses frame, identification of the dominant eye, etc.
Normally a person has a dominant eye and a non-dominant eye. The dominant eye is the eye that has a greater visual acuity and, therefore, dominates the depth vision. The non-dominant eye usually dominates the peripheral and spatial vision. Their interaction causes the brain to receive a three-dimensional image. Usually the dominant eye is the eye that is used to look through a microscope, a camera, or for any task in which only one eye is used.
The parameter relating to the near inter-pupillary distance may be defined as the measurement of the distance between the centers of the pupils of the user when the user is looking at an object that is located at a position near to the eyes of the user.
The parameter relating to the far inter-pupillary distance may be defined as the measurement of the distance between the centers of the pupils of the user when the user is looking at an object that is located at a far position from the eyes of the user.
The parameter relating to the working distance (which may take into account a prescribed power for near vision) may be defined as the distance between the user's eyes and a working area which is habitual/comfortable for the user, such as for example a reading distance.
The parameter relating to the vertex distance (for a given glasses frame) may be defined as the measurement of the distance between the front surface of the eye and the rear surface of the lens mounted on the glasses frame.
All these optometric parameters are widely known and used in the field of optometry, and are often based on standards which make their values to be substantially unambiguously interpreted by the optometric technicians.
In progressive lenses, the position of the power for near vision with respect to the power for far vision in its horizontal displacement is called inset. In conventional progressive lenses, the inset usually has a fixed value accepted as standard. Some of these insets may cause a number of limitations for the ocular convergence in terms of, for example, the working distance and the value of the prescription (power).
Therefore, many users may have difficulties of adaptation to progressive lenses even in the case of having a normal binocular vision and possibly other normal clinical parameters. Clinical cases that may indicate a failure to adapt to progressive lenses may include: strabismus, amblyopia, anisometropia, convergence dysfunction, retinal pathologies, etc.